I've been using excellent tips found here to enhance Firefox performance including cache2ram
I wanted to put not just the cache, but also the Firefox profile, and even other apps on the RAMdisk and some guides were a little too complex for my capabilities. So using a combination of portable apps, Esperance DV, and Carbon Copy Cloner, I'm able to easily run Firefox and VLC from a self-auto-restoring RAMdisk that I can simply eject if I need all of my RAM for work (e.g. CS5).
First download and install Esperance DV
and set up your RAMdisk with the features you require. I checked 1GB, create on opening session, save in disk image (for CCC) and self auto restore. These setting can easily be modified later, thus Esperance DV is recommended.
Download your required portable applications (like Portable Firefox) from FreeSMUG
and install them on the RAMdisk. Download the latest updates to the apps and install them on the RAMdisk as well.
On my RAMdisk the cache is separate from the portable app since I had already used this LifeHacker tip
Lastly, use Carbon Copy Cloner
to backup your RAMdisk to the same .dmg file that Esperance DV is on. Select RAMdisk as the source and for the target 'Choose Disk Image' and select the .dmg file initially made for Esperance DV. At this stage you may choose to make the backup a scheduled, background process.
On SSD-only systems, reducing disk writes is key, so on a newer MacBook Pro, I'd keep the backup on an SD card in the built-in slot; otherwise a tiny USB drive might be a good location.
A script to delete the FF cache on exit would be a great addition to this tip. It's possible that another cloning app may have an option to exclude files that I've overlooked.
I've also had success using the same process with VLC portable, for smoother playback of HD video.
I haven't tested this one.]